1. Field of the Invention
The present invention relates to a motor-driven power steering apparatus.
2. Description of the Related Art
In a motor-driven power steering apparatus, as described in Japanese Patent Application Laid-open No. 6-8839 (patent document 1), an electric motor is driven by a motor driving means, a rotation of the electric motor is converted into a linear stroke of a rack shaft by a power transmission mechanism, and a wheel coupled to the rack shaft is steered and assisted.
It may happen that the electric motor employed in the motor-driven power steering apparatus is used in a wide working temperature range, from a low temperature of, for example, −40° C. or less, to a high temperature of, for example, 100° C. or more, due to an installation position under a hood or the like. An output torque characteristic thereof is changed while exposed to such environmental temperature conditions, so that a magnet is demagnetized toward a higher temperature, and the output torque characteristic is lowered.
On the other hand, in the motor-driven power steering apparatus, a minimum output torque T0 (called as a demand output torque) to be provided in the electric motor is necessary with respect to every traveling condition of a vehicle.
Accordingly, in the prior art, it is necessary to arrange that the output torque characteristic with respect to the temperature dependency of the electric motor becomes equal to or more than the demand output torque T0 in a whole working temperature range to be estimated. In other words, it is necessary to employ an electric motor in which an output torque of a motor taking temperature dependency into consideration is as shown by a line B in FIG. 8.
However, since the output torque characteristic of the temperature dependency of the electric motor shown by line B in FIG. 8 is set such as to secure the demand output torque T0 in the high temperature range, the output torque characteristic is set to an excess specification which exceeds the demand output torque T0 in the low temperature range by itself. Accordingly, it has been conventionally desired that the electric motor satisfies the demand output torque in the whole working temperature range and its capacity is reduced to a minimum limit.